1. Field of the Invention
The present invention relates to an electronic component for surface mounting applications, and in particular to a power supply device including a DC-DC converter.
2. Description of the Related Art
A known power device module for surface mounting applications is illustrated in FIGS. 1a-1d. FIG. 1a is a top view of the device; FIGS. 1b and 1d are side views; and FIG. 1c is an isometric view.
The module includes a generally rectangular transfer moulded housing 10 forming a closed device in which the converter electronics are contained. Terminal connections 11 from the internal electronics pass through the sides and form connecting feet 12 that can be attached to a base board (not shown). The module can be built using a transfer moulded lead frame, allowing the product to be developed with minimum tolerances while maintaining a small size and flat top and bottom sides for ease of handling.
The power device module shown in FIGS. 1a-1d is for surface mounting applications, meaning it is constructed and sold in the form shown in FIGS. 1a-1d for attachment to a circuit board. In practice, a large number of modules are loaded onto a delivery ribbon, which is then fed into an assembly line machine for pick and place processing. The module housing of devices intended for surface mounting applications typically therefore have a flat top side, for protection of the internal components, and so that the assembly line machine can easily pick the module, detach it from the feed ribbon, and place it at the desired location on a circuit board. The external terminal connections are then soldered in place by a further assembly line tool.
Changes in the materials which are considered acceptable for use in manufacture, mandated by the Restriction of Hazardous Substances (RoHS) provisions, for example, have made the reliable manufacture of such devices increasingly difficult. Soldering temperatures of lead-free solder, for example, can be around 80-100° C. greater than the soldering temperatures of solder mixtures that contain lead. When working on encapsulated devices, such as those discussed above, the increased working temperature is now sufficient to cause problems with moisture sensitivity as well as solder reflow, essentially melting solder that has already been applied internally to the module. As a result, problems with the reliability of the device can occur.
One way of dealing with solder reflow problems is to make the housing slightly larger, thereby allowing more heat to escape during the soldering process. However, devices that are small in size are advantageous, as the smaller the footprint of the device when installed on a circuit board, the more devices that can be accommodated. The device shown in FIGS. 1a-1d, for example, would typically be approximately 10 mm in length and 6 mm high. There is therefore generally no desire to increase the size of the device.
An alternative is to move towards what is known as an open frame or a semi-open frame device. This is one in which the housing does not entirely encapsulate the internal electronics, but instead allows space for the flow of air between the interior and the exterior. A power module could, for example, be constructed inside a hollow plastic box. However, because of minimum wall thickness and injection molding tolerances, the final device would necessarily be larger and more complex.
Furthermore, for DC-DC converter modules, the large number of terminations required for the components, typically nine transformer terminations and at least 4 pins for input and output, means that space is at a premium, and the size of the device is limited.
Thus, there is a need for a power module design addressing these problems.
By way of background, the construction of a known DC-DC converter is explained in more detail in Japanese Patent Application JP 62-176197. In brief, a transformer, and first and second circuit boards, including the input and output feed circuits for the transformer, are mounted on a base board. A resin cover is also mounted on the board, enclosing the circuit boards and transformer, to provide protection for the components of the device. Within the cover, the circuit boards are spaced apart, both from one another, and from the transformer, which is sandwiched in between the two circuit boards.
As a result of the space between the components, the heat generated by the circuits and the transformer can easily be dissipated. The resin cover is an electrical insulator and is selected also to have good heat dissipation characteristics.
The device shown in JP 62-176197 is an encapsulated design, not configured for surface mounting.